We have confirmed that in the thermal spraying of practical powder materials, the splat shape changes with increasing substrate temperature to a circular disk shape from a fringe shape with splashing at a critical substrate temperature, Tt. The increase of substrate temperature may accompany a kind of essential change on substrate surface, because the effect is maintained until the substrate is cooled down to room temperature. However, the nature of the substrate surface change due to the heating has not been clearly understood yet. In this study, AISI304 stainless steel was employed as a substrate material, and the substrate was heated in an air atmosphere or laser treated as a pretreatment. Substrate surface topography was analyzed precisely by atomic force microscopy, AFM. We discuss the relationship between surface topography in nano-meter scale and splat morphology. Moreover, in order to evaluate the effect of chemical composition of the substrate surface, gold was coated onto the substrate surface by PVD method after the heat treatment. The effect of adsorbate/condensate on the substrate surface on the flattening behavior of thermal sprayed particles was also verified.